1. Field of the Invention
The present invention relates to a fiber-laser device using an optical fiber to a core of which a laser activation material is added, and an image display using this device.
2. Description of the Related Art
An up-conversion method is available for using a long-wavelength laser light to thereby generate a short-wavelength laser light. The up-conversion method is based on a principle comprising a step (1) in which rare earth ions added to a core of an optical fiber absorb an excited light to enter an excited state, a step (2) in which subsequently the thus excited ions additionally absorb the excited light to be further excited; and a step (3) in which the thus further excited ions transit into a lower energy level state to generate a light having a wavelength smaller than that of the absorbed excited light. A technology regarding the up-conversion is disclosed, for example, in a literature by J. Y. Allain, et al. “Blue Upconversion Fluorozirconate FiberLaser” Electron Lett. 26, 1990, 166”. There is also another literature available by E. W. J. Oomen et al. “A Material and Device Study for Obtaining a Blue Upconversion FiberLaser” Philips J. Res. 46, 157-198, 1992.
Furthermore, Jpn. Pat. Appln. KOKAI Publication No. 7-226551 discloses a method for using Tm ions or Tb ions as rare earth ions to obtain a light having a short wavelength of 455 nm from a laser light having a wavelength of 640-650 nm or 670-810 nm.
According to this method, on both ends of an up-conversion fiber to the core of which the Tm ions or Tb ions are added, films are formed which to have a predetermined reflection coefficient for a 455 nm wavelength. Then, a light having the 455 nm wavelength generated in the fiber resonates between these two films, thus being emitted as a short-wavelength laser light.
The up-conversion fiber is generally elongated (1 to 5 m), depending on an excited-light power, a concentration of added ions, and a fiber core diameter though. As disclosed in the above patent publication, therefore, to form reflection films on both ends of an up-conversion fiber, the fiber must be entirely dipped into, for example, an evaporation tab, which is a problem. This job is actually accompanied by some difficulty in manufacturing. Moreover, an excitation light source becomes high temperature in use, so that an up-conversion fiber, if near the excitation light source, gets heat conducted therefrom. This gives a possibility that a shift of an optical axis owing to fluctuations in temperature may occur at a spliced portion between the excitation light source and the up-conversion fiber.
Thus, a prior art up-conversion fiber has some difficulty in forming reflection films on both ends thereof. Moreover, the up-conversion fiber, if near the excitation light source, may encounter the shift of the optical axis at its splicing owing to the fluctuations in temperature.